Conventional thick film electroluminescent (hereinafter to be referred to as EL) lamps are formed in the manner to be described below. Generally speaking, the term "thick film" EL lamps refers to lamps in which the electroluminescent materials are deposited in films not more than about one and a half to two mils thick. They are distinguished from "thin" film EL lamps in which the films are deposited by evaporative deposition and are in the range of about one to four microns thick.
This invention will be described herein with respect to thick film EL lamp structures. In the conventional construction, the lamp was made by applying successive coatings on an aluminum back electrode. A barium titanate layer is applied first; followed by a phosphor layer, and finally a layer of indium oxide. The barium titanate (such as General Electric Barium Titanate Suspension 117-3-7) and indium oxide (such as General Electric Indium Oxide Suspension 117-3-16) layers are made with commercial suspensions with a CNEC binder (such as supplied by General Electric Corporation). Each coating layer is dried. After drying, lead terminals are attached to the indium oxide layer and aluminum back electrode. The resulting assembly is sealed in a moisture-resistant film, such as a transparent polymer film.
Such a conventional thick film EL structure has the drawback that, when the structure is taken out into the air after drying during the manufacturing process, for the purpose of being sealed, the assembly absorbs the moisture contained in the air; and further that, owing to the moisture located within the device, the service life of the device is markedly reduced. Nevertheless, in spite of these drawbacks, various patents have been granted on the construction of EL lamps including the following:
U.S. Pat. No. 2,838,715--Payne, concerns the construction of an EL lamp comprising a first electrode, a second electrode in close proximity, a solid layer between the electrodes including an electroluminescent phosphor. At least one of the electrodes has a light transmitting conductive solid in close contact within the phosphor layer. U.S. Pat. No. 2,840,741--Lehmann, enhanced the light output of the cell by aligning the phosphor particles in the dielectric.
U.S. Pat. No. 2,944,177--Piper, shows a process and construction for improving the maintenance characteristics of an electroluminescent cell by encapsulating phosphor particles in glass and suspending the encapsulated particles in a dielectric binder. In U.S. Pat. No. 2,951,169--Faria et al., the phosphor is coated with colloidal transparent silica to minimize the decrease in the efficiency of the device with increasing field strength.
U.S. Pat. No. 3,023,338--Cerulli, reveals a copper compound added to copper-activated zinc sulfide phosphor and mixed them with a fine glass frit. When the mix is fired at a temperature lower than required to crystallize the matrix for copper-activated zinc sulfide phosphor, the continuous layer formed electroluminescence evenly and brightly. In U.S. Pat. No. 3,238,407--Jaffe, an improved electroluminescent cell is provided by using a high dielectric binder such as cyanoethyl polyglucoside.
U.S. Pat. No. 3,070,722--Bouchard, discloses a means for reducing the deterioration of the light output of a lamp by providing a hermetically-sealed plastic enclosure for the lamp. U.S. Pat. No. 3,246,193--Dickson, Jr. et al., reveals a continuous layer of electroluminescent phosphor used in a dielectric material applied to a conductive substrate. The second electrode consists of a metallic material such as aluminum, gold, or silver, applied in the desired pattern on the phosphor layer by an acceptable means such as vacuum deposition.
U.S. Pat. No. 3,281,619--Green, reveals the edge terminated type of display device. This insures that the leads to the electrode sections will not capacitively couple to the light-transmitting electrode when a potential is applied. U.S. Pat. No. 3,350,596--Burns, shows a device in which a semi-conductor such as tin chloride is used in both electrodes to even out the field in the phosphor layer and allow a voltage of the order of the breakdown voltage to be applied to the entire layer.
U.S. Pat. No. 4,020,389--Dickson et al., reveals a flexible electroluminescent lamp consisting of a three-layer sandwich of a thin film metal between layers of a thin film dielectric deposited on a polymeric substrate. The phosphor in a suitable binder system was coated on an aluminum substrate to form the other electrode. The assembled members are adhered together by passing them between heated rollers.